00007516562013-10-312013-10-310000751656ck0000751656:S000002529Member2013-10-312013-10-310000751656ck0000751656:S000002529Memberck0000751656:C000006976Member2013-10-312013-10-310000751656ck0000751656:S000002529Memberck0000751656:C000006977Member2013-10-312013-10-31MFS MUNICIPAL SERIES TRUST497false00007516562013-10-312013-10-312013-10-312013-07-29MFS Arkansas Municipal Bond FundMFARXMBARXThe sub-section entitled "Principal Risks" beneath the main heading "Summary of Key Information" is restated in its entirety as follows: Principal Risks<div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">The principal risks of investing in the fund are:</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Interest Rate Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>The price of a debt instrument falls when interest rates rise and rises when interest rates fall. Instruments with longer maturities, or that do not pay current interest, are more sensitive to interest rate changes.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Credit Risk:</font> The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in the financial condition of the issuer, borrower, counterparty, or underlying collateral or assets, or changes in specific or general market, economic, industry, political, regulatory, geopolitical, and other conditions.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">The credit quality of, and the ability to pay principal and interest when due by, an issuer of a municipal instrument depends on the credit quality of the entity supporting the municipal instrument, how essential any services supported by the municipal instrument are, the sufficiency of any revenues or taxes that support the municipal instrument, and/or the willingness or ability of the appropriate government entity to approve any appropriations necessary to support the municipal instrument.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">Lower quality debt instruments (commonly referred to as &#8220;high yield securities&#8221; or &#8220;junk bonds&#8221;) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Lower quality debt instruments are regarded as having predominantly speculative characteristics. Lower quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Geographic Focus Risk:</font> The fund&#8217;s performance will be closely tied to the economic and political conditions in Arkansas, and can be more volatile than the performance of a more geographically diversified fund. In addition, the fund's performance can also be tied to the economic and political conditions of U.S. territories and possessions. These conditions may include constitutional or statutory limits on an issuer&#8217;s ability to raise revenues or increase taxes, anticipated or actual budget deficits or other financial difficulties, or changes in the credit quality of municipal issuers in the state or in U.S. territories and possessions.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Municipal Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>The price of a municipal instrument can be volatile and significantly affected by adverse tax or court rulings, legislative or political changes, changes in specific or general market and economic conditions, and the financial condition of municipal issuers and insurers. Because many municipal instruments are issued to finance similar projects, conditions in these industries can significantly affect the fund and the overall municipal market.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Prepayment/Extension Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument&#8217;s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Derivatives Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives&#8217; original cost.&#160;&#160;Derivatives can involve leverage.</font> </div> <br/><div style="TEXT-INDENT: 0pt; DISPLAY: block"> <br/> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Leveraging Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Investment Selection Risk:</font> MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Counterparty and Third Party Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty&#8217;s or third party&#8217;s ability to perform in accordance with the terms of the transaction.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt"><font style="DISPLAY: inline; FONT-WEIGHT: bold">Liquidity Risk:</font> <font style="FONT-STYLE: italic; DISPLAY: inline">&#160;</font>It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.</font> </div>As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The sub-section entitled "Principal Investment Strategies" beneath the main heading "Summary of Key Information" is restated in its entirety as follows: Principal Investment Strategies<div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from federal income tax and personal income tax, if any, of Arkansas. Interest from the fund&#8217;s investments may be subject to the federal alternative minimum tax.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund&#8217;s assets primarily in municipal instruments.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">MFS may invest 25% or more of the fund&#8217;s assets in municipal instruments that finance similar projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations. Although MFS seeks to invest the fund&#8217;s assets in municipal instruments whose interest is exempt from federal and state personal income tax, MFS may also invest in taxable instruments.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">MFS primarily invests the fund&#8217;s assets in investment grade debt instruments, but may also invest in less than investment grade quality debt instruments (lower quality debt instruments).</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">MFS invests a large percentage of the fund&#8217;s assets in municipal issuers of Arkansas.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, structured securities, inverse floating rate instruments, and swaps.</font> </div> <br/><div align="left" style="TEXT-INDENT: 0pt; DISPLAY: block; MARGIN-LEFT: 0pt; MARGIN-RIGHT: 0pt"> <font style="DISPLAY: inline; FONT-FAMILY: Times New Roman; FONT-SIZE: 10pt">MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative models that systematically evaluate instruments may also be considered. In structuring the fund, MFS may also consider top-down factors.</font> </div><div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"> <font style="display: inline; font-family: Times New Roman; font-size: 10pt; font-weight: bold;">SUPPLEMENT TO PROSPECTUS<br/> </font> </div> <br/><div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"> <font style="display: inline; font-family: Times New Roman; font-size: 10pt; font-weight: bold;">The date of this supplement is October 31, 2013<br/> </font> </div> <br/><div align="left" style="text-indent: 0pt; display: block; margin-left: 0pt; margin-right: 0pt;"> <font style="display: inline; font-family: Times New Roman; font-size: 10pt; font-weight: bold;">MFS&#174; Arkansas Municipal Bond Fund<br/> </font> </div>